Functional parameters of voltage-activated Ca2+ currents from olfactory interneurons in the antennal lobe of Periplaneta americana

Toward our goal to better understand the physiological parameters that mediate olfactory information processing on the cellular level, voltage-activated calcium currents (I-Ca) in olfactory interneurons of the antennal lobe from adult cockroaches were analyzed under two conditions: 1) in acutely dissociated cells (in vitro) and 2) in an intact brain preparation (in situ). The study included an analysis of modulatory effects of potential inorganic and organic Ca2+ channel blockers. I-Ca was isolated and identified using pharmacological, voltage, and ion substitution protocols. I-Ca consisted of two components: transient and sustained. The decay of the transient component was largely Ca2+ dependent. In vitro, I-Ca had an activation threshold of -50 mV with a maximal peak current at -7 mV and a half-maximal voltage (V-0.5act) for tail-current activation of -18 mV. In situ these parameters were significantly shifted to more depolarized membrane potentials: I-Ca activated at -40 mV with a maximal peak current at 8 mV and a V-0.5act for tail-current activation of -11 mV. The sensitivity of I-Ca to the divalent cations Cd2+, Co2+, and Ni2+ was dose dependent. The most effective blocker was Cd2+ with an IC50 of 10(-5) M followed by Ni2+ (IC50 = 3.13 x 10(-3) M) and Co2+ (IC50 = 1.06 x 10(-3) M). The organic channel blockers verapamil, diltiazem, and nifedipine also blocked I-Ca in a dose-dependent way and had differential effects on the current waveform. Verapamil blocked I-Ca with an IC50 of 1.5 x 10(-4) M and diltiazem had an IC50 of 2.87 x 10(-4) M. Nifedipine blocked I-Ca by 33% at a concentration of 10(-4) M.